Issue: No captcha for posting new articles, when unregistered

It's not possible to add new posts/articles to the wiki for "Unregistered / Not Logged In" users, because the security captcha isn't shown.

The post form is displayed correctly, but there is no captcha shown in it and after trying to post the article, an error message, that the char code isn't correct is displayed, but as mentioned there is no field/image, where I could enter the code. When I try to post a new thread in a normal (non-wiki) forum as an unregistered user, the captcha is shown correctly and it works fine.

When I look in the template "vault_edit_main", the variable $human_verify is there, but it's obviously not replaced with the captcha field in the output.

I think, it's a bug or is there a special hint?